The present invention relates to optical fibers. More particularly, it relates to a method for manufacturing a single-mode optical fiber in which the core is formed of laminae, adjacent ones of which have different refractive indices.
In certain applications of single-mode optical waveguide fibers, e.g. gyroscopes, sensors and the like, it is important that the propagating optical signal retain the polarization characteristics of the input light in the presence of external depolarizing perturbations. This requires the fiber to have an azimuthal asymmetry of the refractive index profile. Copending Application Ser. No. 499,898 entitled "Polarization Retaining Single-Mode Optical Waveguide" filed in the name of V. A. Bhagavatula et al. on June 1, 1983 describes a fiber, the core of which is formed of a plurality of laminae of transparent material, adjacent ones of which have different refractive indices. It would be advantageous to form the core of such a fiber by a chemical vapor deposition (CVD) technique so that the fiber is suitable for transmitting optical signals over long distances.
Methods of forming optical fibers having an azimuthal asymmetry of the refractive index profile are disclosed in U.S. Pat. Nos. 3,980,459 (Li) and 3,982,916 (Miller) and in U.S. patent application Ser. No. 500,004, "Method of Forming Optical Fiber Having Laminated Core" (A. Sarkar) filed June 1, 1983. One or more layers are formed on the inner surface of a substrate tube by flowing a reactant vapor through the tube and heating the tube and contained vapor mixture with a moving heat source, external to the substrate tube, such that a suspension of glass particles is produced within the substrate tube, the particles traveling downstream and being fused to form a continuous glassy deposit on the inner surface. An asymmetric structure is formed by inserting a rod into the substrate tube or by asymmetrically heating the substrate tube to preferentially deposit glass particles on a specific region of the tube.